The present invention relates generally to an over-current protection circuit. More particularly, the present invention relates to controlling the amount of current that flows through an analog switch.
Computing devices are used with peripheral devices such as keyboards, mice and printers. A Universal Serial Bus (USB) port is often employed to connect one or more peripheral devices to a computing device. The USB port can be employed for both communicating with a peripheral device and also providing the electrical power that is used to operate the peripheral device. However, when a short to ground occurs, the maximum amount of electrical power provided to the peripheral device must be controlled to prevent damage to the USB port and/or the computing device.
An analog switch is often employed to control the amount of current (power) provided to a peripheral device through a USB port. Typically, an over-current protection scheme for the analog switch is used to prevent damage to the USB port caused by excess current flowing through the port to the peripheral device. Also, the over-current protection scheme can prevent the computing device""s power supply for the USB port from getting loaded down and affecting the proper operation of other components in the computing device. For example, if a paper clip is inadvertently inserted into a USB port, this action could inadvertently cause the computing device""s power supply to short to ground, which may result in significant damage to the power supply and prevent the computing device""s other components from operating properly. Without an over-current protection scheme limiting the amount of current flowing through the analog switch, a fire could result from a hard electrical short to ground.
To provide some level of protection for a computing device, an over-current protection scheme should immediately sense when a hard short to ground occurs, i.e., a relatively large current is drawn through the analog switch. As soon as an excessive current is detected, the scheme causes the analog switch to immediately turn off. In the past, the over-current protection scheme typically entailed placing a chemical fuse in series with the analog switch. The chemical fuse would open the connection whenever excess current was drawn through the analog switch. After a period of time, the chemical fuse would regenerate and close the connection so that current could again flow through the analog switch.
Chemical fuses have several disadvantages. When multiple short circuits occur over a relatively short period of time, the resistance of the fuse can increase. When this condition occurs, a computing device""s power supply may not be able to provide the required voltage to a peripheral device connected to a USB port. Additionally, the performance of a chemical fuse (ability to open a connection and regenerate) can degrade with repeated use over time. Also, the addition of a chemical fuse can increase costs and occupy valuable space in an integrated circuit package.
In accordance with the invention, a multiple over current protection scheme is provided for controlling current through an analog switch that is coupled between a power supply and a load. The scheme includes a first circuit that comprises a comparator for monitoring a voltage drop across the analog switch. The comparator outputs a signal that momentarily activates a first transistor when the voltage drop exceeds a reference voltage. The activated first transistor causes the analog switch to turn off. The scheme also includes a second circuit that comprises a second transistor, an amplifier, and a current source. The amplifier enters a current limiting mode and the second transistor is activated when the voltage drop across the analog switch causes the comparator to output the signal. The amplifier in the current limiting mode maintains a constant voltage at the source of the second transistor and the load side of the analog switch. Also, the current source draws a current through the second transistor. Moreover, the maximum amount of current flowing through the analog switch is controlled by a tracking of the current flowing through the second transistor.
In accordance with another aspect, the invention is directed to including with the first circuit a single-shot pulse generator coupled between the comparator and the first transistor. The comparator triggers the single-shot pulse generator to momentarily activate the first transistor when the voltage drop across the analog switch exceeds the reference voltage.
In accordance with yet another embodiment, the invention is directed to providing a third transistor to operate as the analog switch. The analog switch can be a selected one of MOS, JFET, Bipolar and GaAs semiconductor devices. The drain of the first transistor is coupled to the gate of the third transistor. The third transistor is momentarily turned off when the first transistor is activated.
In accordance with still another embodiment, the invention is directed to providing a third transistor that is physically larger than the second transistor by a factor. The amount of current flowing through the third transistor is limited to the product of the current flowing through the second transistor and a ratio of the physical size differences between the third transistor and the second transistor.
In accordance with another embodiment, the invention is directed to coupling the source of the second transistor and the source of the third transistor to the inputs of the amplifier. Also, the output of the amplifier can be coupled to the gate of the second transistor and the gate of the third transistor.
In accordance with yet another embodiment, the invention is directed to a latch and a switch coupled between the gate of the second transistor and the output of the comparator, which is included with the second circuit. The latch triggers the switch when the comparator is activated causing the amplifier to enter the current limiting mode. The latch can be a selected one of SR, JK and D flip flops. Additionally, the analog switch can provide electrical power to a peripheral device coupled to a USB port. Also, the reference voltage can be approximately 100 milliVolts, the current source outputs approximately 1 milliAmp and the factor approximately 1000.
The invention may also be implemented as methods that perform substantially the same functionality as the embodiments of the invention discussed above and below.
These and other features as well as advantages, which characterize the invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.